Rodriguez-Pinto A.,Geodinamica Interna |
Rodriguez-Pinto A.,Instituto Geologico Y Minero Of Espana |
Pueyo E.L.,Instituto Geologico Y Minero Of Espana |
Barnolas A.,Instituto Geologico Y Minero Of Espana |
And 3 more authors.
Physics of the Earth and Planetary Interiors | Year: 2013
When two or more magnetic components share the same temperature and/or coercivity spectrum in a rock, the paleomagnetic components are said to be overlapping. Failure to separately identify these components in the laboratory may lead to substantial errors in the declination and inclination of the primary component (including apparent changes in polarity and with a marked impact in stability tests). Recently, numerical models have been proposed to understand this problem in relation to fold geometry; these allow the expected declination and inclination errors at different locations across the fold surface to be identified and estimated. In this paper, we present a real case study of overlapped paleomagnetic directions from the northern part of the Balzes anticline (South Central Pyrenees). A new dataset is established based on 206 paleomagnetic samples taken in a wide variety of structural locations. Several facies including limestones, siltstones and marls from Eocene marine and deltaic sediments were studied. This dataset shows the occurrence of paleomagnetic overlapping controlled by lithology. That is, most paleomagnetic directions derived from marls show hardly any significant declination and inclination errors and they are consistent in all structural positions. In contrast, in most of the limestones there is strong overlapping and significant errors related to the fold geometry. The dataset also enables the evaluation of errors as a function of structural position. The numerical model for Balzes anticline overlapped directions shows qualification and quantification of errors in all possible structural positions and the effect of overlaps on the fold test is also analyzed. Through the use of numerical modeling we propose a new tool to deal with data with a strong overlapping of components as in this case study. The identification and quantification of the overlapping ratio potentially enables data to be filtered to eliminate such errors and also allows suitable structural positions where error is minimized to be identified. © 2012 Elsevier B.V.
Rodriguez-Pinto A.,Geodinamica Interna. |
Ramon M.J.,Oficina de Proyectos de Zaragoza |
Oliva-Urcia B.,Geodinamica Interna. |
Pueyo E.L.,Oficina de Proyectos de Zaragoza |
Pocovi A.,Geodinamica Interna.
Physics of the Earth and Planetary Interiors | Year: 2011
The reliability of paleomagnetic data is a keystone to obtain trustable kinematics interpretations. The determination of the real paleomagnetic component recorded at certain time in the geological evolution of a rock can be affected by several sources of errors: inclination shallowing, declination biases caused by incorrect restoration to the ancient field, internal deformation of rock volumes and lack of isolation of the paleomagnetic primary vector during the laboratory procedures (overlapping of components). These errors will limit or impede the validity of paleomagnetism as the only three-dimension reference. This paper presents the first systematic modeling of the effect of overlapped vectors referred to declination, inclination and stability tests taking into account the key variables: orientation of a primary and secondary (overlapped to the primary) vectors, degree of overlapping (intensity ratio of primary and secondary paleomagnetic vectors) and the fold axis orientation and dip of bedding plane. In this way, several scenarios of overlapping have been modeled in different fold geometries considering both polarities and all the variables aforementioned, allowing to calculate the deviations of the vector obtained in the laboratory (overlapped) with respect to the paleomagnetic reference (not overlapped). Observations from the models confirm that declination errors are larger than the inclination ones. In addition to the geometry factor, errors are mainly controlled by the relative magnitude of the primary respect to the secondary component (P/S ratio). We observe larger asymmetries and bigger magnitudes of errors along the fold location if the primary and secondary records have different polarities. If the primary record (declination) and the fold axis orientation are perpendicular (Ω= 90°), errors reach maximum magnitudes and larger asymmetries along the fold surface (different dips). The effect of overlapping in the fold and reversal tests is also qualitatively analyzed in this paper. © 2011 Elsevier B.V.